Endogenous oxidative damage to nuclear DNA, in the form of base damage, apurinic/apyrimidinic abasic sites (AP sites), and strand breaks, occurs rapidly following cerebral ischemia/reperfusion and is an important trigger of ischemic neuronal cell death. DNA base-excision-repair (BER) is the main repair mechanism for oxidative DNA damage in the brain. AP endonuclease (APE) is the rate-limiting enzyme in the BER pathway, which, when overexpressed, can boost BER activity. Recent studies in non-neuronal and neuronal cells have revealed that APE is an essential survival factor under oxidative stress or DNA-damaging insults. The expression and activity of APE are markedly upregulated in the brain after sublethal ischemia, which has been considered to be a contributing mechanism in neuroprotection induced by ischemic preconditioning. APE activity and the APE-dependent overall BER activity instead rapidly decline following lethal ischemic injury, and lead to the accumulation of cytotoxic oxidative DNA lesions. Despite recent evidence for neuroprotection of cultured neurons, the role of APE in ischemic brain injury is currently poorly understood. Using animal models of cerebral ischemia, we have now obtained the first evidence that overexpression of APE protects against ischemic brain injury in vivo, whereas deletion of APE in the brain remarkably exacerbates ischemic injury. Furthermore, we have identified PKCzeta, the atypical isoform of protein kinase C, as a potent endogenous inhibitor of APE. Based on these novel findings, we hypothesize that augmentation of APE activity by gene delivery or inhibiting PKCzeta protects against ischemic brain injury via preventing accumulation of oxidative DNA damage. We propose three specific aims: Aim 1. Test the hypothesis that transgenic overexpression of APE prevents hippocampal cell death induced by transient global ischemia in rats. Aim 2. Test the hypothesis that aberrant activation of PKCzeta mediates ischemic neuronal injury by functionally disabling APE. Aim 3. Test the hypothesis that enhanced DNA repair via PKCzeta inhibition or intracerebral delivery of biologically active APE prevents hippocampal cell death induced by transient global ischemia. Successful completion of this proposed project would yield important information valuable for future development of new therapeutic strategies for stroke.Oxidative damage to genomic DNA is an important mechanism initiating neuron death in the brain after ischemic stroke. The objective of this proposal is to investigate whether molecular or pharmacological manipulations that enhance DNA repair capacity can ameliorate brain injury in animal models of cerebral ischemia. This information will be valuable for future development of new therapeutic strategies for the treatment of stroke.